1. Field of the Invention
This invention relates to jet powered watercraft, especially personal watercraft (“PWC”). More specifically, the invention concerns the jet propulsion system of the watercraft. In particular, the invention is directed to the construction of a water passage at a position upstream of a jet propulsion unit that modulates the amount of water allowed to pass through the water passage.
2. Description of Related Art
Jet powered watercraft have become very popular in recent years for recreational use and for use as transportation in coastal communities. Jet-propelled watercraft offer high performance, improved acceleration and handling, and shallow-water operation. Accordingly, PWCs, which typically employ jet propulsion units, have become popular, especially in resort areas. As the use of PWCs has increased, a desire for improved performance, including greater operational efficiency, also has increased.
Typically, jet powered watercraft, such as PWCs, have a jet propulsion system mounted within the hull that ingests water and expels the water at a high velocity from the stern to propel the watercraft. For directional control, a nozzle is generally provided at the outlet of the jet pump to direct the flow of water in a desired direction. In the conventional PWC, turning is achieved by redirecting the flow of water from the nozzle.
In the typical arrangement for a jet propulsion unit, an engine output shaft is rotationally coupled to a drive shaft. The drive shaft extends into a water passage, which is defined by the hull of the watercraft partially below the water line. The water passage extends from a point forward of the rear of the watercraft to the rear of that watercraft. An impeller is attached to the drive shaft and is disposed within a pump housing portion of the water passage.
FIG. 7 shows a prior art jet propulsion system 600 disposed within the hull 612, of which only a portion is shown in broken lines. As shown, an inlet grate 642 is disposed at an inlet 686 to an intake ramp 688. The inlet grate 642 prevents large rocks, weeds, and other debris from entering the water intake ramp 688 and passing through the jet propulsion system 600. A pump support 650 or ride shoe forms the bottom portion 692 of the water intake ramp 688. The pump support 650 is coupled to the hull 612 within a tunnel 694 through fasteners and/or adhesives (not shown). The pump support 650 includes a main body portion 651 having a vertical attachment surface 652, a forward attachment location 654 that is secured to a ride plate 696, and a ramp portion 656. A passage (not shown) extends through the main body portion 651 of the pump support 650. The ramp portion 656 forms the bottom portion 692 of the water intake ramp 688.
From the water intake ramp 688, water enters into a jet pump 660. The jet pump 660 includes an impeller 670 and a stator 680. The impeller includes blades 672 that extend from a center portion 674 that is coupled to an engine by one or more shafts 698, such as a drive shaft and/or an impeller shaft. The rotation of the impeller 670 pressurizes the water, which then moves over the stator 680 that comprises a plurality of fixed stator blades 682. The role of the stator blades 682 is to decrease the rotational motion of the water so that almost all the energy given to the water is used for thrust, as opposed to just swirling the water. As shown, the impeller 670 and the stator 680 are both disposed within a jet propulsion unit housing 690 or pump housing. However, it is also known to position the stator 680 at a position outside of the housing 690 at a position downstream of the housing 690. The housing 690 includes a peripheral wall 691 which defines a passage through which water passes. A forward end 692 of the housing peripheral wall 691 is attached to the vertical attachment surface 654 or the pump support 650. The forward end 692 of the housing peripheral wall 691 defines the inlet into the housing 690.
Once the water leaves the jet pump 660, it goes through a venturi 610. In this prior art jet propulsion unit 600, the venturi 610 is disposed at the rearward end of the housing 690. Since the venturi's exit diameter is smaller than its entrance diameter, the water is accelerated further, thereby providing more thrust. As shown, the venturi 610 is integrated into the housing 690 and comprises the outlet from the housing 690.
A steering nozzle 602 is pivotally attached to the venturi 610 so as to pivot about a vertical axis 604. The steering nozzle 602 is operatively connected to a steering mechanism such as a steering handlebar (see, e.g., the steering handlebar 74 shown in FIG. 1). Rotation of the steering handlebar causes the steering nozzle 602 to pivot around the vertical axis 604, thereby directing the water discharge to result in a change in the steering direction of the watercraft.
A water passage 695, through which water passes from left to right, is illustrated in FIG. 7. Moving from left to right in this illustration, which is upstream to downstream, the water passage 695 is defined by the inlet 686, the water intake ramp 688, the pump support passage 653, the jet pump 660, the venturi 610 and the steering nozzle 602.
When the amount of water passing through the jet propulsion system 600 is not optimized, it is possible that cavitation may occur as a result of operation of the impeller 670. Cavitation occurs when an object, such as the impeller 670, moves through a fluid, such as water, at a sufficient speed to cause the water to form pockets of vapor. In other words, the impeller 670 can rotate so quickly that, at the tips of the impeller blades 672, a sufficiently low pressure region may be created that the water will flash into vapor, creating small vapor bubbles. When the vapor bubbles collapse, the shock of the collapse can degrade the impeller blades 672 (especially at the tips of the blades 672) by “eating away” at or pitting the blades 672. In addition, cavitation also has the undesired effect of producing noise and vibration that also degrade the operational efficiency of the jet propulsion system 608. In addition, noise and vibration increases the stress and wear and tear on the impeller 670 and components attached thereto.
In addition, when the watercraft is accelerating from a standing still or a low speed condition, the water drawn through the inlet 686 by the action of the pump 660 experiences a drop of static pressure, which is a condition that promotes cavitation. This undesirable drop of pressure can be minimized by increasing the size of the inlet 686, thus optimizing the system for the acceleration mode. Conversely, as the speed of the craft increases, the static pressure in the inlet builds up which leads to a condition that minimizes the formation of cavitation on bubbles in the flow, thus improving the propulsive efficiency of the pump 660. However, when the craft is traveling at high speed, the inlet pressure typically reaches an unnecessarily high level, this being a result of the relatively large inlet size chosen to accommodate the constraints of the low speed acceleration mode. Since a large inlet 686 cuts into the planing area of the hull thus increasing the drag, an inlet 686 optimized for acceleration from low speed will yield lower propulsive efficiency at high speed, while conversely an inlet 686 optimized for high speed will result in poor acceleration performance due to the occurrence of cavitation.
In view of the foregoing, a need has developed for a watercraft with a jet propulsion system that provides improved operational efficiency. Specifically, a need has developed for a watercraft design where the amount of water passing through the jet propulsion system can be modulated.
Also, in view of the foregoing, especially in view of the current trend to increase the operational power of boats and PWCs, a need has developed for a watercraft where cavitation is minimized or eliminated altogether.
These needs remain unaddressed by the prior art.